Dendritic cells (DCs), a heterogeneous population originating from hematopoietic stem cells (HSCs) in the bone marrow (BM), are professional antigen-presenting cells that play key roles in determining the balance between immunity and tolerance induction. Langerhans cells (LCs) are skin-resident DCs that express the C-type lectin Langerin and have a life cycle distinct from many other types of DCs. Even though LCs were first described more than 100 years ago, their development and immunological functions still remain enigmatic. MicroRNAs (miRNAs), a class of 21-25 nt single-stranded non-coding small RNAs, are increasingly being recognized as important regulators of gene expression through the inhibition of effective mRNA translation. The ribonuclease III enzyme Dicer is required for the processing of mature and functional miRNAs. Using Cre-loxP tissue-specific Dicer deletion, our laboratory and others have reported that deletion of miRNAs in HSCs significantly affects the development and function of different immune cells. However, the role of miRNAs in the development of LCs is still currently unknown. To test the roles of miRNAs in the development of LCs, we generated a new mouse strain with tissue- specific disruption of Dicer in LCs using Langerin-Cre. Surprisingly, mice with miRNAs-deficiency in LCs had significantly reduced number of epidermal LCs and the expression of Langerin was significantly reduced in miRNA-deficient LCs. Furthermore, LCs have a specific miRNA gene expression profile compared to other immune cells. Thus, our central hypothesis is that miRNAs are very important components of the molecular circuitry that controls LC development and function. We will test our hypothesis by pursuing the following three Specific Aims: 1) To determine the expression patterns of miRNAs during LC maturation;2) To investigate the role of miRNAs in LC development and function;3) To identify the target genes of miRNAs involved in LC development. Our study will dramatically advance our knowledge on the molecular mechanisms underlying LC development and function, and may also facilitate the development of new intervention strategies for cancer, infectious and autoimmune diseases. PUBLIC HEALTH RELEVANCE: Langerhans cells (LCs) are skin-resident DCs that maintain skin homeostasis, but the detailed molecular pathways regulating LC development and function remain largely unknown. MicroRNAs (miRNAs) are a recently discovered class of evolutionarily conserved small non-coding RNAs that negatively regulate the expression of protein-coding genes. The overall goal of this proposal is to identify the LC-specific miRNA expression profile during development and define their function and direct targets. The results from the above pioneering studies may not only illuminate the new immunological and molecular mechanisms underlying LC development, but may also facilitate the development of new intervention strategies for related autoimmune diseases, infection, and cancer based on the LC cell therapy.